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Explanation:

1) According to the question, the reactivity of a metal is measured by the temperature of the solution after reaction. The higher the temperature of the solution, the greater the reactivity of the metal. Based on this criterion, the metals can be arranged in an order of reactivity as follows;

Mg> Zn > Fe.

2) The equation of the reaction is;

Mg(s) + 2H^+(aq) >Mg^2+(aq) + H2(g)

If we consider the changes in oxidation number from left to right;

The oxidation number of Mg changed from zero to +2

The oxidation number of H changed from +1 to zero

Recall that a redox reaction is a reaction in which the oxidation number of species changes from left to right in the reaction. Hence this reaction is a redox reaction.

The reducing agent experiences an increase in oxidation number. In this case, Mg is the reducing agent.

The oxidation half equation is;

Mg(s) > Mg^2+(aq) + 2e

The reduction half equation is;

2H^+(aq) + 2e > H2(g)